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d="scan'208";a="9639731" Date: Fri, 13 Dec 2019 10:27:42 +0100 From: Roger Pau =?iso-8859-1?Q?Monn=E9?= To: SeongJae Park CC: , , , , , SeongJae Park , , , Subject: Re: Re: [Xen-devel] [PATCH v7 2/3] xen/blkback: Squeeze page pools if a memory pressure is detected Message-ID: <20191213092742.GG11756@Air-de-Roger> References: <20191212152757.GF11756@Air-de-Roger> <20191212160658.10466-1-sj38.park@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset="iso-8859-1" Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <20191212160658.10466-1-sj38.park@gmail.com> X-ClientProxiedBy: AMSPEX02CAS02.citrite.net (10.69.22.113) To AMSPEX02CL03.citrite.net (10.69.22.127) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Dec 12, 2019 at 05:06:58PM +0100, SeongJae Park wrote: > On Thu, 12 Dec 2019 16:27:57 +0100 "Roger Pau Monné" wrote: > > > > diff --git a/drivers/block/xen-blkback/blkback.c b/drivers/block/xen-blkback/blkback.c > > > index fd1e19f1a49f..98823d150905 100644 > > > --- a/drivers/block/xen-blkback/blkback.c > > > +++ b/drivers/block/xen-blkback/blkback.c > > > @@ -142,6 +142,21 @@ static inline bool persistent_gnt_timeout(struct persistent_gnt *persistent_gnt) > > > HZ * xen_blkif_pgrant_timeout); > > > } > > > > > > +/* Once a memory pressure is detected, squeeze free page pools for a while. */ > > > +static unsigned int buffer_squeeze_duration_ms = 10; > > > +module_param_named(buffer_squeeze_duration_ms, > > > + buffer_squeeze_duration_ms, int, 0644); > > > +MODULE_PARM_DESC(buffer_squeeze_duration_ms, > > > +"Duration in ms to squeeze pages buffer when a memory pressure is detected"); > > > + > > > +static unsigned long buffer_squeeze_end; > > > + > > > +void xen_blkbk_reclaim_memory(struct xenbus_device *dev) > > > +{ > > > + buffer_squeeze_end = jiffies + > > > + msecs_to_jiffies(buffer_squeeze_duration_ms); > > > > I'm not sure this is fully correct. This function will be called for > > each blkback instance, but the timeout is stored in a global variable > > that's shared between all blkback instances. Shouldn't this timeout be > > stored in xen_blkif so each instance has it's own local variable? > > > > Or else in the case you have 1k blkback instances the timeout is > > certainly going to be longer than expected, because each call to > > xen_blkbk_reclaim_memory will move it forward. > > Agreed that. I think the extended timeout would not make a visible > performance, though, because the time that 1k-loop take would be short enough > to be ignored compared to the millisecond-scope duration. > > I took this way because I wanted to minimize such structural changes as far as > I can, as this is just a point-fix rather than ultimate solution. That said, > it is not fully correct and very confusing. My another colleague also pointed > out it in internal review. Correct solution would be to adding a variable in > the struct as you suggested or avoiding duplicated update of the variable by > initializing the variable once the squeezing duration passes. I would prefer > the later way, as it is more straightforward and still not introducing > structural change. For example, it might be like below: > > diff --git a/drivers/block/xen-blkback/blkback.c b/drivers/block/xen-blkback/blkback.c > index f41c698dd854..6856c8ef88de 100644 > --- a/drivers/block/xen-blkback/blkback.c > +++ b/drivers/block/xen-blkback/blkback.c > @@ -152,8 +152,9 @@ static unsigned long buffer_squeeze_end; > > void xen_blkbk_reclaim_memory(struct xenbus_device *dev) > { > - buffer_squeeze_end = jiffies + > - msecs_to_jiffies(buffer_squeeze_duration_ms); > + if (!buffer_squeeze_end) > + buffer_squeeze_end = jiffies + > + msecs_to_jiffies(buffer_squeeze_duration_ms); > } > > static inline int get_free_page(struct xen_blkif_ring *ring, struct page **page) > @@ -669,10 +670,13 @@ int xen_blkif_schedule(void *arg) > } > > /* Shrink the free pages pool if it is too large. */ > - if (time_before(jiffies, buffer_squeeze_end)) > + if (time_before(jiffies, buffer_squeeze_end)) { > shrink_free_pagepool(ring, 0); > - else > + } else { > + if (unlikely(buffer_squeeze_end)) > + buffer_squeeze_end = 0; > shrink_free_pagepool(ring, max_buffer_pages); > + } > > if (log_stats && time_after(jiffies, ring->st_print)) > print_stats(ring); > > May I ask you what way would you prefer? I'm not particularly found of this approach, as I think it's racy. Ie: you would have to add some kind of lock to make sure the contents of buffer_squeeze_end stay unmodified during the read and set cycle, or else xen_blkif_schedule will race with xen_blkbk_reclaim_memory. This is likely not a big deal ATM since the code will work as expected in most cases AFAICT, but I would still prefer to have a per-instance buffer_squeeze_end added to xen_blkif, given that the callback is per-instance. I wouldn't call it a structural change, it's just adding a variable to a struct instead of having a shared one, but the code is almost the same as the current version. Thanks, Roger.